title: Impaired growth of denervated muscle contributes to contracture formation following neonatal brachial plexus injury.
abstract: BACKGROUND
  The etiology of shoulder and elbow contractures following neonatal brachial plexus injury is incompletely understood. With use of a mouse model, the current study tests the novel hypothesis that reduced growth of denervated muscle contributes to contractures following neonatal brachial plexus injury.
  
  
  METHODS
  Unilateral brachial plexus injuries were created in neonatal mice by supraclavicular C5-C6 nerve root excision. Shoulder and elbow range of motion was measured four weeks after injury. Fibrosis, cross-sectional area, and functional length of the biceps, brachialis, and subscapularis muscles were measured over four weeks following injury. Muscle satellite cells were cultured from denervated and control biceps muscles to assess myogenic capability. In a comparison group, shoulder motion and subscapularis length were assessed following surgical excision of external rotator muscles.
  
  
  RESULTS
  Shoulder internal rotation and elbow flexion contractures developed on the involved side within four weeks following brachial plexus injury. Excision of the biceps and brachialis muscles relieved the elbow flexion contractures. The biceps muscles were histologically fibrotic, whereas fatty infiltration predominated in the brachialis and rotator cuff muscles. The biceps and brachialis muscles displayed reduced cross-sectional and longitudinal growth compared with the contralateral muscles. The upper subscapularis muscle similarly displayed reduced longitudinal growth, with the subscapularis shortening correlating with internal rotation contracture. However, excision of the external rotators without brachial plexus injury caused no contractures or subscapularis shortening. Myogenically capable satellite cells were present in denervated biceps muscles despite impaired muscle growth in vivo.
  
  
  CONCLUSIONS
  Injury of the upper trunk of the brachial plexus leads to impaired growth of the biceps and brachialis muscles, which are responsible for elbow flexion contractures, and impaired growth of the subscapularis muscle, which correlates with internal rotation contracture of the shoulder. Shoulder muscle imbalance alone causes neither subscapularis shortening nor internal rotation contracture. Impaired muscle growth cannot be explained solely by absence of functioning satellite cells.
authors.affiliation: Division of Pediatric Orthopaedic Surgery, Cincinnati Children's Hospital Medical Center, 3333 Burnet Avenue, MLC 2017, Cincinnati, OH 45229, USA.
authors.forename: Sia
authors.initials: S
authors.lastname: Nikolaou
authors.forename: Elizabeth
authors.initials: E
authors.lastname: Peterson
authors.forename: Annie
authors.initials: A
authors.lastname: Kim
authors.forename: Christopher
authors.initials: C
authors.lastname: Wylie
authors.forename: Roger
authors.initials: R
authors.lastname: Cornwall
mesh_terms: D000818:Animals; D001917:Brachial Plexus / Q000293:injuries*; D020516:Brachial Plexus Neuropathies / Q000150:complications*; D004195:Disease Models, Animal; D005552:Forelimb; D051379:Mice; D009119:Muscle Contraction; D018482:Muscle, Skeletal / Q000254:growth & development* / Q000294:innervation*; D009135:Muscular Diseases / Q000209:etiology; D012782:Shoulder